The story of possible fossils
from Mars begins with a meteorite, a martian meteorite, found in Antarctica.
Meteorites fall to Earth all the time, but very few are found and collected.
Even fewer are observed falling and landing on the ground. So how can
you tell that a rock is a meteorite if you don't see it fall?

The first thing to
look for in a possible meteorite is a fusion
crust, a thin layer of black or dark glass that coats the meteorite
as if it had been dipped in tar. This meteorite is covered with a black
fusion crust, except where it has been broken off to show the meteorite's
light gray interior.

The fusion crust
forms when the meteorite blazes through our atmosphere on its way from
interplanetary space to Earth. We see the light of this passage as a meteor
streak in the sky. Most pieces of interplanetary dust or rock make meteors
but not meteorites — they burn up completely in the air. As a meteorite
blasts through the air, it gets so hot that its outside melts; when the
meteorite cools down before landing, the melted rock solidifies to glass,
which is the fusion crust. Freshly fallen meteorites have a black, glassy
fusion crust, as do most meteorites collected from Antarctica. The Antarctic
cold and dryness preserve the glass; elsewhere on Earth, the fusion crust
weathers rapidly to become dull and rusty brown, and can look just like
many Earth rocks.

Meteorites come in
many different varieties. Iron meteorites
are probably most familiar; they are typically large and make impressive
museum displays. But nearly 90% of all meteorites are
stony — made of the same materials and minerals as Earth rocks.
But unlike Earth rocks, nearly all meteorites contain some iron metal.
Most stony meteorites are classified
as chondrites (like this meteorite,
ALH 90411). They are made of millimeter- sized balls and clumps of minerals
that formed directly from the gas and dust of the early solar nebula.
The rest of the stony meteorites are called achondrites
(like ALH 84001). They do not contain the millimeter-sized balls and clumps
of ancient minerals; rather, they are
igneous rocks, formed from molten lava on planets or large asteroids.

ALH 90411, a
chondrite meteorite collected in 1990 from the Allan Hills area of Antarctica.
The cube at the lower left is 1 centimeter on a side. S91-38433, NASA/JSC.